Light subject to calibration is typically calibrated in wavelength by comparing its wavelength to an optical wavelength reference. Some types of optical wavelength references provide reference light in which an accurately-defined reference wavelength is marked. The reference wavelength is typically marked by a peak or a dip in the spectrum of the reference light at the reference wavelength. Based on such comparison, the optical calibration system determines the difference between the wavelength of the light subject to calibration and the reference wavelength in the reference light. To calibrate the wavelength of the light subject to calibration, the wavelength of the light subject to calibration is adjusted to match the reference wavelength in response to the wavelength difference.
Two common methods for generating the reference light are atomic emission and molecular absorption. In atomic emission, the reference light is generated by exciting atoms of a gas sealed in a hermetic chamber. Light emitted via excitation of the gas atoms has a particular wavelength, i.e., the reference wavelength, that depends on the type of gas contained in the chamber.
In molecular absorption, light having a range of wavelengths, e.g., white light, broad-band ultra-violet light, etc., is passed through a gas sealed in a hermetic chamber. The gas molecules absorb light from the light passing through the chamber at one or more wavelengths that depend on the type of gas contained in the chamber. Thus, the spectrum of the light after passing through the chamber has an absorption line at each of the one or more wavelengths. One of these wavelengths is designated as the reference wavelength.
Generating reference light using either atomic emission or molecular absorption employs a device having a hermetic chamber containing a particular gas for marking the reference wavelength. Such a device, sometimes referred to as a “gas lamp,” is typically expensive. Moreover, the reference light generated by such a device has relatively few reference wavelengths. Additionally, the reference wavelengths depend on the type of gas contained in the hermetic chamber. The reference wavelengths available may not be conveniently close in wavelength to the desired calibration wavelength. Moreover, the reference wavelengths cannot easily be changed.
Thus, an unaddressed need exists for an optical wavelength standard that is lower in cost than a conventional optical wavelength standard and that and is capable of providing a greater number of reference wavelengths in a given wavelength range than a conventional optical wavelength standard.